1. Field of Invention
This invention relates to a coil structure, and more particularly to a printed coil having improved magnetic coupling, low loss and improved high frequency characteristics, when used as a transformer.
2. Description of the Prior Art
Transformers are widely known and are used as a magnetic component for electronic devices and power units. The conventional transformer comprises an insulator gap between a primary coil and a secondary coil, and the voltage generated in the secondary coil is determined by the voltage applied to the primary coil multiplied by the winding ratio therebetween.
FIG. 1 is a partially cut-away perspective view of a conventional transformer, wherein bobbin 1 is molded by an insulator resin or the like, and ring shaped collar sections 1b are created at both ends of a tubular cylindrical section 1a. Winding section 2 comprises conductive wires 2 wound around cylindrical section 1a of bobbin 1, wherein a primary winding 2a and secondary winding 2b form a double layer with an insulating tape disposed therebetween. Barriers 4 are provided in bobbin 1 in order to form a gap between windings 2 and collar section 1b to satisfy safety standards, and are constructed by winding two layers of tape shaped insulator with insulating tape 3 therebetween. A core 5, which may be an EE type core, is made of magnetic material and has a middle leg 5b, which penetrates through cylindrical section 1a of bobbin 1, and two outer legs 5a positioned on both sides of middle leg 5b. A closed magnetic path is formed by combining two of the EE type cores 5 to improve electromagnetic coupling of the transformer.
However, because wire 2 is wound around cylindrical bobbin 1 in the conventional transformer, there are problems, such as, the winding operation is cumbersome and the device is physically large since bobbin 1, which comprises most of the volume of the transformer, is itself large. Furthermore, barriers 4 are needed because insulation must be fully maintained at the lateral ends of the winding 2 in order to satisfy safety standards. Also, because the radial surroundings of winding 2 are not covered by an insulator, a gap must be provided for insulation.
A device which uses a simplified winding operation is disclosed in Japan UM Laid-Open No. 4/46,524, and is shown in FIGS. 2A and 2B, wherein FIG. 2A depicts a sectional view, and FIG. 2B depicts a perspective view, of a simplex stack bobbin 6. Several stack bobbins 6 are laminated together and a core 5 is attached thereto and form a transformer. A plate insulating barrier 7 is attached at the boundary between the primary side and secondary side of stack bobbins 6. Insulating covers 8 are attached to the outsides of stack bobbins 6.
Stack bobbin 6 has a plate 6a which is a partition between the layers of windings 2 and a cylindrical magnetic core section 6b having a rectangular opening provided at the center of plate 6a. Two pull out guide sections 6c are provided at both ends of the lower end of plate 6a to keep plate 6a at a predetermined position. A pin section 6d is provided on the pull out guide section 6c, which is soldered to a printed board (not shown), and forms a terminal to which winding 2 is connected. When plates 6a are to be stacked, they may be disposed in a telescopic manner so that pull out guide sections 6c will not interfere with each other. Winding 2 is wound about magnetic core section 6b and both ends thereof are connected to pin sections 6d. Core 5 has a middle leg 2 which is disposed through magnetic core sections 6b.
The winding operation involves running the wire along plate 6a and about magnetic core section 6b. Thus, as compared to the case where winding 2 is wound around a cylindrical bobbin, such as in FIG. 1, the winding operation is simplified.
However, because the lateral and radial surroundings of winding 2, are not covered by an insulator, a gap necessary to provide insulation is needed. Thus, as with FIG. 1, the problem of size of the transformer remains. Furthermore, because the number of pin sections 6d increases corresponding to the number of laminations of the stack bobbins 6, when a telescopic structure is adopted for the pull out guide section 6c, the winding operation for wiring around each pin section 6d or for wiring between each pin section 6d, becomes complicated.
Moreover, because the primary coil and secondary coil are separately laminated on stack bobbins 6, only the plane on which insulating barrier 7 is provided becomes the magnetic coupling plane of the primary and second windings, thereby increasing leakage inductance and degrading magnetic coupling between the primary winding and the secondary winding. Moreover, effective AC resistance significantly increases by the so-called proximity effect when there is a conductor in which a high frequency current flows in the same direction. Also, resistance increases when the winding direction on each plate of the stack bobbins 6 is such that current flows in the same direction.
As for floating capacity, there is a problem between adjacent plates of the stack bobbins 6. If a commercial power source is connected to the primary side, the voltage on the primary side is 100V to 220V and if the secondary side is used for driving a logic circuit, its voltage is 5V to 15V. That is, the primary voltage is higher than the secondary voltage by a factor of about one digit, that is a factor of 10. Because electrostatic energy is proportional to the square of voltage,the floating capacity of stack bobbins 6, used as the primary coil, becomes 100 times that of the secondary coil, if the transformation ratio of the transformer is 10:1.